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United States Patent |
5,539,667
|
Rode
|
July 23, 1996
|
Method and apparatus for improved digital film recorder
Abstract
The invention discloses a method and apparatus for an improved digital film
recorder. The improved recorder is capable of quantizing and recording
high resolution images, in order to reduce recording time, without adding,
or displaying appreciable quantization error artifacts. An image processor
is provided which takes an image value at a select one of M input levels
and quantizes the image value down to a select one of N output levels
where M>N. The image processor of the invention then generates an error
value which is related to the deviation between an idealized input signal
and the quantized output signal. This error signal is then added back into
the next input signal in order to smooth, or diffuse the error across a
large area. Finally, in order to further reduce visible artifacts
associated with the repetitive errors accompanying image areas having
little or no change in image value, noise may be intentionally injected in
order to break up any "bunching" of errors.
Inventors:
|
Rode; Christian S. (Waltham, MA)
|
Assignee:
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GCC Technologies (Bedford, MA)
|
Appl. No.:
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945120 |
Filed:
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September 15, 1992 |
Current U.S. Class: |
386/40; 345/428; 386/42 |
Intern'l Class: |
G06T 005/00 |
Field of Search: |
364/525,526,514,517
395/128,131,132
345/147
|
References Cited
U.S. Patent Documents
2960019 | Nov., 1960 | Craig.
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3881098 | Apr., 1975 | Rich | 235/151.
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3984187 | Oct., 1976 | Bestenreiner et al. | 355/80.
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4134668 | Jan., 1979 | Coburn | 355/3.
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4457618 | Jul., 1984 | Plummer | 355/20.
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4654721 | Mar., 1987 | Goertzel et al. | 358/283.
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4668995 | May., 1987 | Chen et al. | 358/282.
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4701045 | Oct., 1987 | Plummer | 355/20.
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4712909 | Dec., 1987 | Oshikoshi | 355/20.
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4723166 | Feb., 1988 | Stratton | 358/167.
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4739374 | Apr., 1988 | Mead et al. | 355/67.
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4764807 | Aug., 1988 | Kimura et al. | 358/75.
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4849809 | Jul., 1989 | Thara et al. | 358/75.
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4855940 | Aug., 1989 | Richardson et al. | 364/526.
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4891714 | Jan., 1990 | Klees | 358/456.
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4903068 | Feb., 1990 | Shiota | 355/20.
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4930018 | May., 1990 | Chan et al. | 358/298.
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4941013 | Jul., 1990 | Hara | 355/20.
|
4947204 | Aug., 1990 | Endo | 355/20.
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4958238 | Sep., 1990 | Katayama et al. | 358/456.
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4962542 | Oct., 1990 | Klees | 382/54.
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4969052 | Nov., 1990 | Ishida et al. | 358/457.
|
4972189 | Nov., 1990 | Polito et al. | 341/118.
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4975786 | Dec., 1990 | Katayama et al. | 358/459.
|
4996530 | Feb., 1991 | Hilton | 341/120.
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5014333 | May., 1991 | Miller et al | 382/54.
|
5016040 | May., 1991 | Dwyer, III | 355/20.
|
5031050 | Jul., 1991 | Chan | 358/298.
|
5034990 | Jul., 1991 | Klees | 382/22.
|
5036398 | Jul., 1991 | Westell | 358/214.
|
5045952 | Sep., 1991 | Eschbach | 358/447.
|
5050000 | Sep., 1991 | Ng | 358/298.
|
5051841 | Sep., 1991 | Bowers wt al. | 358/447.
|
5051844 | Sep., 1991 | Sullivan | 358/456.
|
5060284 | Oct., 1991 | Klees | 382/53.
|
5070515 | Dec., 1991 | Iwahashi et al. | 375/27.
|
Other References
Foley & VanDam-Fundamentals of Interactive Computer Graphics 1982
Edition-pp. 597-602.
Foley & VanDam-Fundamentals of Interactive Computer Graphics 1990-Second
Edition-pp. 572-573.
Pratt-Digital Image Processing-pp. 616-625 1978-CH-22, Digital Point
Processing Image Coding.
|
Primary Examiner: Ramirez; Ellis B.
Assistant Examiner: Pipala; Edward
Attorney, Agent or Firm: Goldstein; Robin Diane
Claims
What is claimed is:
1. An improved digital film recorder, for use in recording an image present
on an electronic display device to a photographic emulsion, the
improvement comprising:
an improved digital film recorder mechanism;
interface means, comprising at least a first input and a first output, for
receiving an image signal at said first input representative of a value of
a portion of an image being displayed on a display device, said image
signal being provided to said interface means first output at a select one
of M values;
noise generation means for generating a noise signal;
first combination means for combining said image signal present at said
first output of said interface means with the noise signal generated by
said noise generation means and supplying the resulting combined signal to
a first combination means output; and
quantization means for quantizing the signal present at said first
combination means output, said quantization means having at least a
quantized output, the operation of said quantization means being such that
said quantized output is at a select one of N values, where M>N;
wherein said improved digital film recorder mechanism records the image
being displayed on said display device to a photographic emulsion by
recording the quantized output combination of said image signal and said
noise signal.
2. The improved digital film recorder, as claimed in claim 1, wherein said
image signal comprises at least a luminance component and a chrominance
component; said first combination means acting to combine only said
luminance component of said image signal with said noise signal; and
wherein said improved digital film recorder records the image being
displayed on said display device to a photographic emulsion by recording
the quantized output combination of said noise signal and the luminance
component of said image signal.
3. The improved digital film recorder, as claimed in claim 1, wherein said
noise signal is supplied as a digital noise signal capable of being at one
of a multiplicity of values; and
wherein said improved digital film recorder records the image being
displayed on said display device to a photographic emulsion by recording
the quantized output combination of said image signal and said digital
noise signal.
4. The improved digital film recorder, as claimed in claim 3, wherein said
image signal comprises data representative of a single color plane of a
multi color image being displayed on said display device;
said digital noise signal being controllable such that the value of the
digital noise signal applied to a same point of an image in each color
plane of a multi color image is the same; and
wherein said improved digital film recorder mechanism records a single
color plane of the multi color image being displayed on said display
device to a photographic emulsion by recording the quantized output
combination of said image signal and said digital noise signal.
5. An improved digital film recorder, for use in recording an image, the
improvement comprising:
an improved digital film recorder mechanism;
interface means, comprising at least a first input and a first output, for
receiving an image signal representative of a value of a portion of an
image being displayed on a display device, said image signal being at a
select one of M values, said interface means further comprising an image
delay means interposed between said first input and said first output for
receiving and selectively storing said image signals;
noise generation means for generating a noise signal;
quantization means for quantizing said image signal, said quantization
means comprising a first input coupled to the first output of said image
input means, a second input for receiving said noise signal and a third
input;
said quantization means further comprising a quantized output, the
operation of said quantization means being such that said quantized output
is at a select one of N values, where M>N;
said quantization means additionally comprising an error output for
outputting an error signal, said error signal being representative of the
deviation of said quantized output from said first input of said
quantization means; and
error delay means for receiving and storing said error signal, said error
delay means comprising an error delay input coupled to said error output
of said quantization means, said error delay means further comprising an
error delay output for selectively providing said delayed error signal to
the third input of said quantization means;
whereby the quantized output of said quantization means is effectively
modified such that any repetitive quantization error is diminished; and
wherein said improved digital film recorder mechanism records the image
being displayed on said display device to a photographic emulsion by
recording the quantized output combination of said image signal, said
noise signal and said delayed error signal.
6. The improved digital film recorder, as claimed in claim 5, wherein said
image signal comprises at least a luminance component and a chrominance
component; said quantization means acting to combine only said luminance
component of said input signal with said noise signal;
wherein said improved digital film recorder mechanism records the image
being displayed on said display device to a photographic emulsion by
recording the quantized output combination of the luminance component of
said image signal, said noise signal and said delayed error signal.
7. The improved digital film recorder, as claimed in claim 5, wherein said
noise signal is supplied as a digital noise signal; and
wherein said improved digital film recorder mechanism records the image
being displayed on said display device to a photographic emulsion by
recording the quantized output combination of said image signal, said
digital noise signal and said delayed error signal.
8. The improved digital film recorder, as claimed in claim 7, wherein said
image signal represents the data contained in a single color plane of a
multi color image capable of being displayed on said display device;
said digital noise signal being controllable such that the digital noise
value applied to a same point of an image in each color plane is the same;
wherein said improved digital film recorder mechanism records a single
color plane of the image being displayed on said display device to a
photographic emulsion by recording the quantized output combination of
said image signal and said digital noise signal.
9. The improved digital film recorder, as claimed in claim 5, wherein said
image signal may be represented as a digital value X bits long.
10. The improved digital film recorder, as claimed in claim 9, wherein said
quantized output from said quantization means may be represented as a
digital value Y bits long.
11. The improved digital film recorder, as claimed in claim 10, wherein
said quantized output from said quantization means comprises the Y most
significant bits of said X bit long image signal.
12. The improved digital film recorder, as claimed in claim 10, wherein
said error signal may be represented as a digital value comprising the X-Y
least significant bits of said image signal.
13. The improved digital film recorder, as claimed in claim 9 wherein X=8.
14. The improved digital film recorder, as claimed in claim 10, wherein
Y=5.
15. The improved digital film recorder, as claimed in claim 12, wherein
said X-Y least significant bits represent a two's complement value.
16. An improved method for digitally recording an image on film, the
improved method comprising the steps of:
receiving an image signal representative of a value of a portion of an
image to be recorded, said input being at a select one of M values;
generating a noise signal;
combining said image signal with said noise signal;
quantizing the resulting combination of said image signal and said noise
signal to a quantized output signal, such that said quantized output
signal is at a select one of N values, where M>N; and
recording said quantized output signal on to a photographic emulsion.
17. The improved method for digitally recording an image on film, as
claimed in claim 16, wherein said noise signal is generated as a digital
noise signal.
18. The improved method for digitally recording an image on film, as
claimed in claim 17, wherein said image signal represents the data of a
single color plane of a multi color image being displayed on a display
device;
said digital noise signal being controllable such that the digital noise
value applied to a same point of an image in each color plane is the same.
19. An improved digital film recorder, for use in recording an image, the
improvement comprising:
an improved digital film recorder mechanism;
interface means, comprising at least a first input and a first output, for
receiving an image signal at said first input, said image signal being
representative of a value of a portion of an image being displayed on a
display device, said image signal being supplied at a select one of M
values at said first output;
noise generation means for generating a noise signal; and
quantization means for quantizing said image signal, said quantization
means comprising a noise input for receiving said noise signal and a first
input coupled to the first output of said interface means and wherein said
quantization means further comprises:
a quantized output, the operation of said quantization means being such
that said quantized output is at a select one of N values, where M>N; and
an error output for outputting at least an error signal, said error signal
being representative of the deviation of the value of the signal at said
quantized output from said value of the signal at the first input of said
quantization means;
wherein said improved digital film recorder mechanism records the image
being displayed on said display device to a photographic emulsion by
recording the quantized output of said image signal and said noise signal.
Description
BACKGROUND OF THE INVENTION
This invention relates, generally, to the field of digital film recorders,
and more particularly to an improved digital film recorder which may
provide increased recording speed while minimizing the introduction of
visible and undesirable artifacts associated with the recording of a
quantized image.
In the fields of computers and electronics, a series of sophisticated tools
have been developed which permit individuals to display and manipulate
images on a personal computer or work station. In particular, a number of
image scanners and other input devices have been developed which allow
full-color, photographic-quality images to be incorporated into computer
applications.
Along with such image scanners, new software packages have also been
developed with which a user may generate, on a computer, full color images
consisting of charts, graphics, and the like, which may then be
incorporated into printed documents, viewed on a video display, or
projected as a form of electronic slide show.
Improvements have also been made in the performance of color display
devices. These new display systems have the capability to render
electronic images recorded at resolutions of 1024.times.768 pixels or
higher. By using software which can develop and process images in a 24-bit
color space, these input and display systems allow users to work with
images made up of over 16.7 million colors.
While much use may be made of such high resolution and high color density
scanned and computer-generated images in association with high quality
display devices, it is often desirable to transform such screen images
into hard copy representations which may then be used or manipulated
further using traditional photographic means. Positive prints and slides
are two such examples of the kinds of hard copy output which are often
desirable in commercial, professional and engineering environments.
Therefore, to transfer and record the high quality images generated on a
computer and displayed on its screen to a more permanent and fixed form,
film recorders have been developed.
Early efforts at film recorders were often simple variations on the theme
of pointing a camera at a cathode ray tube or other suitable display
device and then taking a picture. However, film recorders have since been
developed which are capable of producing a more accurate end product by
assembling an image on photographic film in a digital manner. Such digital
image manipulation and recording techniques provide more precise and
repeatable control to the user and offer a system which can produce a more
accurate and pleasing finished photographic product.
An example of one such digital film recorder is a device described in U.S.
Pat. No. 4,855,940 issued to Thomas L. Richardson, et al. on Aug. 8, 1989
and assigned to the Polaroid Corporation of Cambridge, Mass.; the
teachings of which are incorporated herein by reference. In the '940
patent, Richardson describes a method and apparatus for defining and
photographing computer images using a photographic exposure adaptor and
providing an interface between a computer having a CPU and a film printer.
Unfortunately, the apparatus disclosed by Richardson suffers from a number
of limitations which reduce its overall usefulness. These limitations and
their solutions are the focus of the present invention.
In particular, as discussed in Richardson, a digital film recorder may
produce an exposure on a plate of light-sensitive film of an image to be
recorded by repeatedly scanning each line of the image, which is displayed
on a cathode ray tube or other image-producing device. This repetitive
scanning is accomplished with a beam which is modulated in a binary manner
such that the cumulative exposure of the light sensitive film by the beam
produces a gray scale image. More particularly, given a known film speed,
in order to produce an image which is capable of resolving 256 gray
levels, at least 256 scans must be performed, with at least one scan for
each possible gray level. Assuming that each scan will take approximately
50 milliseconds to expose a line of linear film, the complete recording of
a 640.times.480 pixel image made up of 256 grey levels will take almost
half a minute. Using a similar scan rate, the same recording at a
resolution of 2048.times.1366 pixels would take over a minute.
Extrapolating further to a color system, then, we find that if the
recording of a color image having a color resolution of 8 bits per primary
color is desired, at least 256 scans must be performed for each red, blue
and green color component yielding a minimum of 768 scans in order to
produce a fully exposed 24-bit color image. It is noted that although
24-bit color is not the same as continuous analog color, it is, in fact,
accurate enough for all but the most critical work.
Therefore, based on the above assumptions, at an output resolution of
2048.times.1366 pixels, a recording will require over four minutes, which
is far too long for high volume output. In actual practice more than 256
per color may be required, since most film, color or monochromatic, has
non-linearities in the properties of each recording layer. In addition, to
match the typically non-linear input space of the input image, additional
corrections, such as "gamma corrections" will also often need to be made.
There will, therefore, be the need to compensate for these non-linearities
in the exposure process by exposing some levels more than once in order to
get 256 levels of nearly linear output.
Since the total scanning time is a function of (1) the number of exposure
levels multiplied by (2) the number of lines scanned divided by (3) the
number of lines scanned per second, one way to speed up the scanning
process is to reduce the number of exposure levels which are recorded by
the film recorder. However, while reducing the number of exposure levels
from 256 per color component (or 256 in total for a monochrome image) to a
smaller number will result in a faster overall recording time, quantizing
from an input of 256 levels, which requires eight bits of information, to
an output of fewer levels will result in an increased quantization error,
and the increased production of visual artifacts. In addition, since in
practice more scans must be performed for each output level in order to
properly expose the photographic film as well as to compensate for various
non-linear qualities, such as the film or the image input space, the use
of fewer scans will result in a photographic film which receives less
total exposure and is not properly compensated for its non-linearities.
Therefore, it has been determined that the need exists for an improved
method and apparatus for digital film recording which overcomes the above
noted limitations and which provides for decreased scan times by reducing
the number of scans required for each color component, as well as reducing
the required scan rate, while significantly reducing the problems
associated with image quantization and maintaining sufficient image
illumination.
SUMMARY OF THE INVENTION
This invention discloses a method and apparatus for an improved film
recorder. In general, it is recognized that while the recording of
quantized "natural" images may produce visually acceptable results, the
recording of quantized computer-generated images, such as those generated
by presentation graphics programs, will entail the quantization of images
with smoothly changing or unchanging image areas and boundaries. This is
because such computer generated images tend to have more ramps and more
saturated colors and more abrupt image transitions than natural images.
Unfortunately, it has been noted that the resulting quantization of these
unchanging, or smoothly changing areas and boundaries contribute to the
production of visible errors, which substantially degrade the quality of
any hard copy output.
Therefore, generally speaking, in accordance with the invention, a method
and apparatus for improved digital film recording is provided which can
accurately record a 24-bit color image (or 8-bit monochromatic image)
using fewer bits for the recording of each color without adding or
displaying appreciable image quantization artifacts and without suffering
the attendant film exposure problems found in the prior art.
In one application of the invention, a 256-level, or 8-bit monochromatic,
image is provided as an input to the invention. Quantization error, which
is a measure of the deviation of the actual output from the idealized
output, is then added to the next input signal. Finally, a random noise
signal may be added to the input signal after the addition of the last
error value, and this new combination may then be passed to the quantizing
function. In one embodiment, the incoming signal is quantized to a
32-level output, with the five most significant bits of the input
comprising the quantization output and the three least significant bits of
the input indicating the aforenoted error data.
By using such an architecture, a 256-level input signal may be quantized to
a 32-level output signal with the resulting quantization error being
retained and used to more accurately quantize the signal. In addition,
since the quantization of an unchanging or slightly changing input signal
may result in visible and unwanted artifacts, a noise signal having an
absolute magnitude which is preferably not greater than the maximum value
of the truncated portion of the quantized input signal may be added to the
input signal to diffuse and disperse any added quantization errors and to
reduce unwanted visual artifacts in the output image due to repeating
quantization error.
Finally, due to the reduced number of scans the reduction of illumination
available to expose the light-sensitive film may be compensated for
through the use of a faster film and/or a moderate increase in the
modulated beam intensity.
Accordingly, it is an object of the invention to provide an improved
apparatus for digital film recording which overcomes the limitations of
the prior art.
It is another object of the invention to provide an improved method for
digital film recording which overcomes the limitations of the prior art.
It is a further object of the invention to provide an improved method and
apparatus for digital film recording which can record digital images using
quantization while minimizing or eliminating the recording of excessive
image artifacts.
It is still another object of the invention to provide an improved method
and apparatus for digital film recording in which quantization errors are
diffused to render them less visible.
It is still a further object of the invention to provide an improved method
and apparatus for digital film recording whereby a random level of noise
not exceeding a predetermined value is added to a signal to be recorded in
order to distribute quantization errors across at least a portion of the
output signal and lessen their visual impact.
It is still an additional object of the invention to provide an improved
method and apparatus for digital film recording which employs the
diffusion of quantization errors and the use of added noise.
It is yet another object of the invention to provide an improved method and
apparatus for digital film recording which may record color images at a
speed which is faster than that allowed by traditional recording methods
while reducing or eliminating any newly-introduced associated data
quantization errors.
It is even another object of the invention to provide an improved method
and apparatus for digital film recording which may compensate for the
decreased exposure provided to a photographic film due to a decrease in
the total number of scans.
It is yet a further object of the invention to provide an improved method
and apparatus for digital film recording which may employ the process of
scanning a CRT for the purpose of recording an image, wherein the scanning
rate may be reduced in order to permit the employment of a simpler and
less expensive CRT.
It is even a further object of the invention to provide an improved method
and apparatus for digital film recording which is inexpensive and easy to
assemble.
Still other objects and advantages of the invention will, in part, be
obvious and will, in part, be apparent from the specification.
The invention accordingly comprises the features of construction,
combinations of elements and arrangement of parts which will be
exemplified in the detailed descriptions hereinafter set forth, and the
scope of the invention will be indicated in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the invention, reference is had to the
following descriptions taken in connection with the accompanying drawings
in which:
FIG. 1 is a block diagram illustrating the quantization of an input signal
at one of 256 levels to an output signal at one of 32 levels with
resulting artifacts;
FIG. 2 is a block diagram illustrating the quantization of an input signal
at one of 256 levels to an output signal at one of 32 levels, wherein the
quantization error output is added to the next of said input signals;
FIG. 3 is a block diagram illustrating the quantization of an input signal
at one of 256 levels to an output signal at one of 32 levels, wherein
noise is added to the input signal;
FIG. 4a is a block diagram illustrating the invention;
FIG. 4b is a block diagram illustrating an embodiment of the invention of
FIG. 4a, wherein an input signal at one of 256 levels is quantized to an
output signal at one of 32 levels;
FIGS. 5a, 5b, 5c, 5d, 5e and 5f, read together, illustrate an embodiment of
the process of the instant invention implemented in the computer language
C;
FIGS. 6a and 6b, combine to form a chart illustrating input signals, output
signals and error signals provided in accordance with one embodiment of
the instant invention;
FIG. 7a is an enlarged section of the chart of FIGS. 6a and 6b;
FIG. 7b is a chart illustrating output signals and error signals when the
input signal is at a steady state;
FIG. 8 is an image recorded at 256 grey levels;
FIG. 9 is an image recorded at 256 grey levels and quantized to 32 grey
levels;
FIG. 10 is an image recorded at 256 grey levels and quantized to 32 grey
levels wherein the quantization error is diffused;
FIG. 11 is an image recorded at 256 grey levels and quantized to 32 grey
levels wherein noise is added to the image; and
FIG. 12 is an image recorded at 256 grey levels and quantized to 32 grey
levels wherein the quantization error is diffused and noise is added to
the image.
NOTATION AND NOMENCLATURE
The detailed description that follows is presented largely in terms of
algorithms and symbolic representations of operations on data bits and
data structures within a computer memory. These algorithmic descriptions
and representations are the means used by those skilled in the data
processing arts to most effectively convey in substance their work to
others skilled in the art.
An algorithm is here, and generally, conceived to be a self-consistent
sequence of steps leading to a desired result. These steps are those
requiring physical manipulations of physical quantities. Usually, though
not necessarily, these quantities take the form of electrical or magnetic
signals capable of being stored, transferred, combined, compared, and
otherwise manipulated. It proves convenient at times, principally for
reasons of common usage, to refer to these signals as bit patterns,
values, elements, symbols, characters, data packages, or the like. It
should be borne in mind, however, that all of these and similar terms are
to be associated with the appropriate physical quantities and are merely
convenient labels applied to these quantities.
Further, the manipulations performed are often referred to in terms, such
as adding or comparing, that are commonly associated with mental
operations performed by a human operator. No such capability of a human
operator is necessary, or desirable in most cases, in any of the
operations described herein that form part of the present invention; the
operations are machine operations. Useful machines for performing the
operations of the present invention include general purpose digital
computers or other similar devices. In all cases there should be borne in
mind the distinction between the method of operations in operating a
computer and the method of computation itself. The present invention
relates to method steps for operating a computer in processing electrical
or other (e.g. mechanical, chemical) physical signals to generate other
desired physical signals.
The present invention also relates to an apparatus for performing these
operations. This apparatus may be specially constructed for the required
purposes, or it may comprise a general purpose computer as selectively
activated or reconfigured by a computer program stored in the computer.
The algorithms presented herein are not inherently related to any
particular computer or other apparatus. In particular, various general
purpose machines may be used with programs written in accordance with the
teachings herein, or it may prove more convenient to construct a more
specialized apparatus to perform the required method steps. The required
structure for a variety of these machines will appear from the description
given below.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a simplified block diagram of the quantization of an
input signal at one of 256 levels (such as a point in the image of FIG. 8)
to an output signal at one of 32 levels (such as the same point in the
image of FIG. 9) is shown. For the purposes of describing the instant
invention, it is assumed that the input to the system is a signal which
may be represented as being at one of 256 levels. In a binary system, this
input signal may, therefore, be represented by an eight bit value.
Further, if the input signal represents 8-bits of resolution of a single
color, then using the well-known RGB (red, green, blue) color model, eight
bits of red, eight bits of green, and eight bits of blue color information
may each be represented, resulting in a 24 bit color space which may
define over sixteen million colors.
It is noted that in the description of the present invention, the output
signal is defined to be at one of 32 levels. However, it is recognized
that the input signal may be quantized to an output signal having any
number of levels, such as 16 or 64. In addition, it is also recognized
that while the embodiment of the invention in the instant application is
described as quantizing an image from 256 levels to 32 levels, quantizing
from an input signal of 256 levels to an output signal of 33 levels will
allow the invention to be practiced using a simple and a computationally
efficient algorithm, resulting in an improved apparatus which may be
easily and inexpensively manufactured.
Returning to FIG. 1, as can be seen (and as can also be observed in FIG. 8
and FIG. 9,) if an input signal at one of 256 levels is quantized to an
output signal at one of 32 levels, then (depending on the nature of the
input signal), it is very likely that visual information contained in the
input signal will be lost and the resulting quantized output signal will
be a "coarse" or "chunky" representation of the input signal. This is
because the process of quantization is a process which sorts input data
into a "best fit" output data channel. Therefore, in quantizing from an
input signal at one of 256 levels to an output signal at one of 32 levels,
only some inputs will map perfectly to an output channel. However, most
inputs will not map perfectly, but will map approximately to an output
data channel with some positive or negative deviation or error. This
error, derived from the conversion of a signal at one of 256 levels to one
of 32 levels is known as quantization error. In practice, this
quantization error may or may not be objectionable depending upon a number
of factors including the nature and level of the input signal over a
period and whether or not the quantization of the input signal causes
multiple, but different, input signals to "bunch up" or overlap into the
same output channel. If such "overlapping" occurs frequently, the result
will be a quantized output signal which contains visible artifacts. These
artifacts are well known in the image processing area and appear in FIG.
9.
Referring, then, to FIG. 2, a quantization process employing error
diffusion is shown. This process is quite similar to the process shown in
FIG. 1 except that the output of the quantization processor is compared to
the input signal to arrive at a difference, or error signal. An image
created in accordance with the process of FIG. 2 is shown in FIG. 10. As
discussed earlier, if we imagine that the 32 possible output levels of the
quantization processor can be represented by the five most significant
bits of the 8-bit input signal, then the error signal may be understood,
in one embodiment, to be indicated by the least significant three bits of
the input signal which are not otherwise output from the quantization
process. These three least significant bits represent the error, or
missing resolution information, which is not available in the output of
the quantization process. As can be seen by comparing FIG. 9 to FIG. 10,
by adding the resulting quantization error from the instant quantization
process to the next input signal, the output error will be diffused,
across the input signal space. This, in turn, will help to reduce those
boundary or quantization errors which are otherwise recognizable as visual
artifacts.
However, while this error diffusion process is helpful in reducing the
"channeling" or "banding" associated with image quantization, it is not
always a wholly satisfactory solution in the processing of certain types
of computer generated graphic information especially the type generated by
presentation graphics computer programs. This is because, under certain
circumstances, when an input signal remains fairly constant over an
extended period of time, if the quantization error which is generated is
added to the next input signal, it will propagate in a repeating fashion,
thereby generating further visible artifacts. Experience has shown that
such artifacts are especially noticeable in those areas of an image where
there is little change in the input signal, such as areas having long
unbroken boundaries, or areas of long monotonic ramps. In these
situations, the use of the quantization error signal fed back and combined
with the input signal is less helpful than it otherwise would be since,
although the quantization errors will be spread over a large area, the
actual error diffusion pattern will be observable as an objectionable
artifact. As noted, this is especially true for computer generated images
which tend to be "perfect" with well defined and regular image areas and
borders, as opposed to "natural" images which are inherently noisy or
which are typically generated by noisy devices. The effect may be seen in
FIG. 10, wherein repetitive error diffusion patterns may be observed along
the boundaries between adjacent but different output levels.
Referring now to FIGS. 6a and 6b, a table is provided which shows the
relationship of an input to the quantization processor, such as the kind
which might be used with the instant invention, to the output, including
the management of errors associated with quantization overflow, so that
the artifacts associated with error diffusion may be better understood. In
the table of FIGS. 6a and 6b, the input to the system will be at one of
256 levels while the output will be at one of 32 levels. Depending upon
the processing of the output signal, it may be quantized to a value of
between 0 and 31 or, if desired, it may be quantized and scaled to fit
within the same range of values as the 256 level input signal. Therefore,
in the OUTPUT column of FIGS. 6a and 6b, both non-scaled (0-31) and scaled
(0-255) quantized outputs are shown. The ERROR column shows the difference
between the INPUT and the scaled output.
As noted above, in a typical quantization processor, the input signal may
have a value ranging from a white saturation level of 0 to a black
saturation level of 255. In addition, although it is anticipated that the
input may be limited to keep it from having a value which is lower than 0
and higher than 255, the processor is designed to allow the input to fall
outside this range without deleterious effect. In operation, inputs having
a value from 0 through 3 are mapped to an output of 0 (0) while inputs
having a value from 251 through 255 are mapped to an output of 31 (255).
These two output levels of 0 (0) and 31 (255) define the limits of the
processor output. In between these input limits of 0 through 3 and 251
through 255, inputs to the processor in the range of 4 through 250, in
increments of approximately 8, will each increase the output by 1 (8). For
example, when the input ranges between 4 and 11 the output will be 1 (8).
When the input ranges between 12 and 19, the output will be 2 (16), and so
on.
In addition to the overall scaling, it is also desirable that the error
output of the quantization processor be centered in approximately the
middle of each output channel range, with the exception of an output of 0
(0), which will always have a non-negative error and an output of 31
(255), which will always have a non-positive error, so that the
quantization of an input idealized signal in the center of a quantization
band will result in no error output. This output and error scaling can be
easily seen though the use of an example in which the input ranges from 4
and 11. In such an example, the output will always be 1 (8), while the
quantization error will range between an error of +3 for an input of 11;
an error of -4 for an input of 4; and an error of 0 for an input of 8.
By using such a system, quantization errors, which as noted above are added
back into the input data stream, provide a method for more faithfully
converting the overall image from input to output. Therefore, while inputs
of between 4 and 11 will always result in a quantization process output of
1 (8), the associated quantization error output will range between -4 and
+3 thereby affecting subsequent inputs (and in turn, outputs) by tending
to move cumulative inputs which are at the upper end of a quantization
output band to the next higher output and those at the lower end to the
next lower output.
However, as discussed above, if the input signal remains constant for even
a small period of time, then secondary quantization artifacts may appear.
Referring to FIG. 7a and 7b, by way of example, it can be seen that if an
input signal is supplied having a value of 11, which is near the high side
of quantization output 1 (8), then the quantization output will take a
value of 1 (8) while the quantization error output will take a value of
+3. If the next input signal again has a value of 11, then, according to
the error diffusion process shown in FIG. 2, the actual input to the
quantization process will have a value of 14, which is equal to the input
signal of 11 plus the previous quantization error of +3. This will result
in the process output having a value of 2 (16), with an associated
quantization error output of -2. This, in turn, will be added to the next
input signal, once again having a value of 11, which will bring the value
of the actual input to the quantization processor to 9, which will result
in a process output having a value of 1 with an associated quantization
error output +1, and so on.
Unfortunately, as can be seen in FIG. 7b, in the preceding example if the
input remains at a steady state for 8 inputs or more, a highly likely
condition for computer generated images, then the error output signal will
begin to repeat and become visibly apparent. Therefore, an alternative
method is necessary to diffuse steady state inputs to a quantization
processor.
One alternate method of diffusing such steady state inputs is to combine
the input with some small amount of noise before it is provided to
quantization processor. Such a process is shown in FIG. 3, and the output
of a quantized image made in accordance with the process of FIG. 3 is
provided in FIG. 11. As can be seen in comparing the simply quantized
image of FIG. 9 to the "noise enhanced" quantized image of FIG. 11, the
addition of noise to the input signal will, indeed, help to break up the
bunching associated with a single input value over an extended period. In
FIG. 11, it can been seen that the clearly identifiable quantization bands
of FIG. 9 are diminished as a small amount of noise is added to the input
signal prior to quantization. Clearly, the amount of noise added to the
input signal will determine the effectiveness of the process shown in FIG.
3. If, for example, too little noise is added (i.e. 1 part in 256), then,
depending on the content of the image, the diffusion provided by the
additional noise might not be enough to visibly reduce unwanted artifacts.
On the other hand, if enough noise is added to the input signal, it would
be possible to completely obscure the input to the point where no
recognizable image was produced. Therefore, the appropriate amount of
noise to be added in order to diffuse quantization error must be
determined for each image based on the contents of the image. Through
research it has been determined that a noise signal ranging from zero to
the maximum value of that part of the input signal which is truncated by
the quantization process will often provide acceptable results.
However, whereas the error diffusion process illustrated in FIG. 2 and the
addition of noise illustrated in FIG. 3 each provide some value in
reducing image artifacts from a quantized image which is to be recorded by
a digital film recorder, it has been determined that maximum benefit may
be derived by combining the above two techniques.
Therefore, turning to FIG. 4a, a block diagram illustrating the
architecture of the present invention is provided. In the invention, an
image input is provided to an input delay, which may comprise a digital
filter. This filter is preferably of the type which may be modeled as a
finite impulse response (FIR) filter. In such an FIR filter, the input is
provided to an input delay line, which is comprised of multiple delay
elements. Associated with each delay element is a tap, which may be
assigned a weighted value. These weighted outputs are then summed into a
summing node, which produces the output of the digital filter. In
practice, by choosing appropriate tap weightings, any one of a number of
filtering effects may be realized, including the implementation of a low
pass filter, a high pass filter, or a band pass filter.
As shown, the output of any selected number of weighted delay taps are then
provided as inputs to a quantization function. The quantization function
produces a quantized output and at least one error output as a function of
delayed input signals, a noise input and the feedback of a delayed
quantization function error output. In practice what happens within the
quantization function may be adjusted by modifying the parameters of the
input delay, the noise input, and the error delay. However, the
quantization function may be modeled as:
output.sub.0 =f.sub.Q (input.sub.0, input.sub.1, input.sub.2, . . . ,
error.sub.1, error.sub.2, . . . , noise)
error.sub.0 =f.sub.err (input.sub.0, input.sub.1, input.sub.2, . . . ,
error.sub.1, error.sub.2, . . . , noise)
where output.sub.0 is the quantization output; error.sub.0 is the
quantization error output; input.sub.0, 1 and 2 . . . are tap weighted
inputs which may be in the form of delayed inputs; noise is random
information preferably having a value within the signal input space; and
error.sub.1,2 and 3 . . . error outputs from other quantization steps.
By employing such a model, any n-dimensional model of the invention may be
developed. By way of example, by using a model with two-dimensional
quantization, multiple weighted tap inputs may be used so that input.sub.0
may be the current pixel multiplied by 50% while input.sub.1 may be a
previous pixel to the left of input.sub.0 multiplied by 25% and
input.sub.2 may be a previous pixel above input.sub.0 multiplied by 25%.
Another example of such weighting is the Floyd Steinberg algorithm where
the concept of two-dimensional error diffusion is practiced by using 3/8
of a pixel to the right of the current pixel, 3/8 of a pixel below the
current pixel and 2/8 of a pixel down and to the right of the current
pixel. This weighting scheme can also be implemented in the present
invention.
Turning next to FIG. 4b, a block diagram of a specific implementation of
the instant invention illustrated in FIG. 4a is shown in which the
aforenoted limitations of quantization, and quantization coupled with
error diffusion or quantization coupled with noise are significantly
eliminated. The results of a process performed according to FIG. 4b are
illustrated in FIG. 12.
The quantization function shown in FIG. 4b may be modeled as:
output.sub.0 =Q(input.sub.0 +error.sub.1 +noise)
error.sub.0 =input.sub.0 +error.sub.1 -output.sub.0
where Q is the quantize or truncation operator.
Under the instant embodiment of the invention, an input signal representing
a point in an image to be recorded, and at one of 256 levels, or eight
bits, is provided. This input signal is then combined with the
quantization error signal calculated and carried over from the last
quantization process in a manner similar to that shown in FIG. 2, and
described in detail above.
A noise signal is then added to this combination in a manner similar to
that shown in FIG. 3, also as described above. As discussed earlier, while
the noise signal may have any value, it may be readily understood that a
noise signal with a large maximum value will unnecessarily obscure details
present in the original input signal, while a noise signal with a small
maximum value may not be sufficient to accomplish the results desired
through the practice of the instant invention. Therefore, as noted, by
experimentation, it has been determined that a noise signal having a
maximum value no greater than the portion of the input signal which is
truncated by the quantization process will provide the desired results.
By way of example, if our 256-level input signal can be represented in
eight bits, and the quantization process will truncate the three least
significant bits of the input signal to produce an output 32 level output
signal which can be represented in five bits, then the noise signal should
preferably have a maximum value which may be represented in no more than
three bits. Using the two's complement numbering system, these three least
significant bits will have an absolute value of between 0 and 7 or a two's
complement value of between +3 and -4. In practice it is preferred to use
the two's complement value, since it will allow the invention generated
noise signal to be both positive and negative instead of residing totally
in the positive or negative noise domain. This total combination is then
quantized by the quantization processor which, once again, yields an
output which may be at one of 32 levels. This 32 level output is then also
compared to the present input before the addition of noise but after the
addition of the previous error to arrive at a value which is
representative of the deviation of the actual output signal from an
idealized output signal had the 256 level input signal been perfectly
converted to a 32 level value with no error.
Therefore, by employing the processes shown in FIG. 4a or FIG. 4b, and
described above, a quantized output being at one of 32 levels may be
provided in which the quantization error is not lost, but is fed back as
an additional input to the quantization process in order to spread any
quantization errors over a long period of input signals, and, further,
where internally generated noise is also added to the input signal, in
order to "break up" any "bunching" or overlapping of quantization error
which may result from a slowly changing or steady state input signal.
Referring to FIGS. 5a, 5b, 5c, 5d, 5e and 5f, a computationally efficient
algorithm implemented in the "C" computer language is shown which scales a
256 level input down to a 32 level output including the output of an error
value, and which adds a noise signal to the input signal.
Thus, as can be seen, an improved method and apparatus for a digital film
recorder may be provided which overcomes limitations of the prior art and
which may be easily and inexpensively implemented. As a result, thereof,
an image capable of being displayed at a high resolution and in color on a
video display may be recorded on light sensitive film in an efficient and
relatively rapid manner without adding or displaying appreciable
quantization error artifacts.
It is appreciated that by using an implementation of the invention the
recording film will receive less exposure than would otherwise be the
case. However, it is recognized that such exposure deficiencies may be
compensated for by using a film with a higher speed. While such higher
speed film may have a grain pattern which is more noticeable, in fact
through experimentation it has also been determined that the film grain
even at higher speeds is still smaller than the grain of the phosphor on
the recording CRT.
It is further appreciated that while the above described embodiment of the
invention utilizes an image generation source which is coupled to a
display, it is possible to generate computer images at resolutions which
are higher than those displayable on such a display. In such a case the
image generator may be connected only to the digital film recorder of the
invention and no display used. Similarly, when such an image display is
used, although it is, for the purposes of this invention, described as
being of the traditional cathode ray tube variety, any suitable display
technology, color or monochromatic, may be used. This is equally true for
the recording CRT employed by the film recorder, where such recording CRT
may also be capable of directly displaying a color or monochromatic image,
and where such recording CRT may not be an actual CRT, but may utilize any
alternate display technology, such as liquid crystal display technology.
It is also appreciated that while the specific example of a system having
an input which may take a value of between 0 and 255 and a scaled output
which may take a value of between 0 and 31 is shown by way of example, any
other desired scaling function may also be implemented as desired. There
is no limitation on the number of input levels or output levels or on the
relationship between the input and output scaling under this invention.
There is also no limitation on the scaling of the error output signal or
its absolute relationship to either the input signal value or output
signal value. In the example given in this application, it is desirable to
maintain the value of the output between 0 and 31. This is done by
establishing the scaling output to fall between 0 and 31 for any input,
and by never allowing an output with a value of 0 to have a negative error
and by never allowing an output with a value of 31 to have a positive
error. However, this design choice may be easily modified according to the
needs of the information which is being processed.
It is additionally appreciated that while conversion of the input signal to
the output signal and the generation of intermediate error terms may be
done with look-up tables, such a conversion may also be performed in a
computationally efficient manner without look-up tables through the use of
digital logic, devices such as programmable logic arrays, field
programmable gate arrays, programmable logic devices or custom silicon.
It is appreciated, as well, that the use of a noise signal having a value
which has an absolute value which does not exceed the absolute value of
the least significant bits which have been truncated though the
quantization process is merely a design choice and not rigidly required.
It is understood that, depending upon the nature of the input signal, a
smaller noise level may provide acceptable results and be sufficient to
mask any repetitive quantization errors. It is also recognized that a
sufficiently long repeating input string moderated by a relatively small
noise amount may also yield some visible quantization artifacts so that in
such a case the noise input will have to be increased.
With respect to the added noise of the invention, such noise is preferably
inserted in the digital domain since the image processing itself takes
place completely in the digital domain and thus no noise is inherent in
the system. However, it is anticipated that analog components may be used
which have an inherent noise element and that such inherent noise element
may be combined with the input signal and the error signal in accordance
with the teachings of the invention to mask quantization errors. It is
also understood that the invention may be practiced without the use of the
quantization error signal if the noise signal is large enough. Finally, it
is anticipated that while the examples given above have been with
reference to single color plane of a three color system, the invention
will work equally well for a monochromatic system as for a full-color
system which is composed of more than three color planes.
Additionally, it is noted that while for optimum masking results all noise
added to an embodiment of the invention to be practiced in a multi-color
system should be "random", in order to minimize the addition of color
artifacts along with the introduction of noise, the noise which is added
to each color plane during processing should be correlated. In this way,
while the noise will be random across the image space, the noise will be
consistent across the three color spaces. This desired result may be
realized through the use of "pseudo-random" noise, or noise which may be
repeated through the use of seed keys or other mechanism.
Finally, it is noted that while the invention disclosed above is discussed
with respect to a linear input space and a linear output space,
modifications such as input and output correction tables may be made to
the quantization processor, the input delay processor, and the error delay
processor to adjust for non-linear input and non-linear output spaces.
Applied to FIG. 4a, this would mean that the quantization function can
process a linear input space into a non-linear output space by keeping
track of the closest match of the desired value from the input space,
without reference to the actual quantization process, which produces the
best fit to the output space. It would then save and feed back the linear
error term as an addition to the next input.
Accordingly, it will thus be seen that the objects set forth above, among
those made apparent from the preceding description, are efficiently
attained and, since certain changes may be made in the above constructions
without departing from the spirit and scope of the invention, it is
intended that all of the matter contained in the above description or
shown in the accompanying drawings, shall be interpreted as illustrative,
and not in a limiting sense.
It will also be understood that the following claims are intended to cover
all of the generic and specific features of the invention, herein
described, and all statements of the scope of the invention which, as a
matter of language, might be said to fall there between.
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